Engineering ingenuity20 August 2012
Once described as a drowning machine, Calgary Bow River weir could not be removed due to the strategic importance it plays in supplying irrigation water. Now, however, it is heralded as an engineering success.
There’s more to the Calgary Bow River weir project than meets the eye. Its well earned accolades speak for themselves. Located within Canada’s Ingelwood District near the City of Calgary, the scheme has been described as an excellent example of ‘engineering ingenuity’; and ‘a collaborative engineering approach to solve a highly complex problem’. Furthermore, this revered project has also enhanced and enriched its local community and the environment. So what’s the story behind its success?
In 1908 the Western Irrigation District Headworks were constructed by the Canadian Pacific Railroad to divert water from the Bow River to the semi-arid region east of Calgary. As it still is today, the diverted water was used for irrigation and livestock, as well as playing a pivotal role in the development of the surrounding areas.
Located on the Bow River the original headworks were constructed of timber, being replaced in 1924 with reinforced concrete structures. In 1975 the headworks were replaced again to include:
• A headgate structure which diverts water from the Bow River to the Western Irrigation District.
• A 152.4m long and 2.6m high ogee weir structure (known as Calgary weir).
• A sluiceway structure to divert the Bow River round the weir in the autumn and winter.
• A vertical slot fishway located on the river left (north) end of the weir.
Problems arose as Calgary weir is the only barrier to boaters and fish passage along this 100km section of the Bow River, between the Bearspaw dam and the Calgary-Bow River Headworks east of Calgary. Although a fish ladder had been incorporated at the weir in 1975, differences were identified between fish populations up and downstream of the structure. Indications were that the 1.5m wide vertical slot fishway was not adequately facilitating successful upstream fish passage on the 180m wide river.
The Calgary weir had also become labelled as a drowning machine: since 1975 over 20 people had died here. The weir incorporated a hazardous ogee weir profile, that created a submerged roller or re-circulating hydraulic jump to reduce energy and downstream water velocities. Once people had gone over the weir it was extremely difficult to swim out of the re-circulating hydraulic.
So from the outset the primary purpose of the Calgary Bow River Weir Project was to redevelop the weir to eliminate the extreme drowning hazard that it had created. It also had to enable river passage for non-motorised boats and fish, while maintaining the Western Headwork’s primary function of facilitating water diversion from the Bow River to the Western Irrigation District. As a result of this function the weir could not be removed, only modified. Furthermore, modifications to the weir could not result in increased river levels upstream of the weir during river floods.
Parks Foundation Calgary (PFC) is a non-profit organisation dedicated to the creation of parks, the preservation of river valleys, and the support of amateur sport. It was also instrumental in the planning, vision and fundraising for the project.
PFC commissioned the idea of alternative design concepts to modify the weir. Their conceptual design study concluded that a ‘full width man-made rapid’ was a technically viable concept that offered the following significant benefits:
• Increased safety and navigability.
• Improved fish passage and habitat.
• Reduced costs for Alberta Environment safety measures.
• Reduced costs for the City of Calgary Fire Department Aquatic Rescue Unit.
• New recreational opportunities for white water boating.
• Economic opportunities created by increased tourism to Calgary and surrounding area.
• Aesthetic improvements.
• River, environment and engineering educational opportunities.
Golder Associates, in association with Northwest Hydraulic Consultants (NHC) and Recreation Engineering and Planning (REP) undertook the preliminary designs. Further developing the full width rapid concept NHC carried out physical model studies to confirm hydraulic and safety performance and irrigation diversion capability; as well as assessing non-motorised boat and fish passage. REP provided expert input to the project configuration with emphasis on the design of the white water play features. The results of the physical model studies were used to develop final designs and modifications to the existing structures.
In 2006, along with NHC and REP, klohn-crippen Berger was retained to undertake the final design, planning and construction of the project.
Klohn Crippen Berger soon got to work and constructed drop structures downstream of the weir to improve safety and facilitate better river passage. These create a pool and riffle sequence resulting in higher water levels downstream of the weir. The hydraulics over the weir are changed from a re-circulating hydraulic jump to a lower energy standing wave. The riffle geometry was designed to provide white water play areas primarily for kayaking which had the added bonus of making the safety improvements an amenity for the City of Calgary.
Two channels were constructed immediately downstream of the weir. The low water channel (LWC) on the right side of the river (facing downstream) consists of six concreted boulder drop structures and five constructed pools to provide a bypass channel for novice boaters to pass downstream. This channel is rated as Class II rapids by the boaters. The high water channel (HWC), located in the centre and left side of the river, consists of five concreted boulder drop structures and three constructed pools. The HWC provides white water play areas for more experienced boaters (Class III rapids). The LWC and HWC are separated by a constructed divide island which provides bird habitat and refuge for boaters.
Fish passage enhancements include notches cut into the existing weir to facilitate boat and fish passage at low river flows, and lower velocity fish passage channels formed into the boulder-concreted surface of the drop structures within the LWC and HWC. The vertical slot fishway wall was also extended 60m downstream to confine the pool downstream of the weir.
The islands and concreted-boulder drop structures are relatively low profile (ie do not project very high above the river) and are designed to be overtopped when the river stage exceeds the 1:2 year flood event; this will ensure that flood levels upstream of the weir in downtown Calgary are not increased by the weir modifications.
The hydraulic design of the project utilised a 1:50 scale comprehensive physical model, a 1:15/1:22 scale flume physical model, a 1:12 scale section physical model and 2D numeric computer modelling to assess overall:
• Project hydraulics – such as water levels, velocities, flow patterns, flood conveyance and irrigation diversion delivery.
• Drop structure hydraulics - velocities, flow patterns, fish passage, playwaves, and scour potential.
Physical model testing was performed at NHC’s hydraulic laboratory in Edmonton, Alberta and at the university of Alberta Hydraulics Laboratory in Ellerslie. A comprehensive physical model of the entire project was constructed at a 1:50 scale. It reproduced an 850m reach of the Bow River, extending approximately 350m upstream and 500m downstream of the existing weir. The model reproduced all pertinent features of the existing weir, canal headgate structure, sluiceway structure and river channel bathymetry.
This model was used to validate and compare pre-project and post-project conditions for river discharges varying from a low flow of 44m3 up to and above the 1:100 year flood discharge of 2750m3. In addition to the comprehensive models, flume models of individual LWC and HWC drop structures were constructed at scales of 1:15 for the LWC drops, and 1:22 scale and 1:12 scale for the HWC drops. These provided more detailed indication of the hydraulic performance of the drop structures and were capable of reproducing river discharges ranging from the low river flow of 44m3 to the 1:5 year flood discharge of 725m3.
The hydraulic design replicated in the comprehensive physical model formed the basis for the sizing, geometry and location of the drop structures, constructed pools and islands. A comprehensive digital terrain model was created of the hydraulic model which helped with construction drawings, quantities and construction layout. The digital terrain model was subsequently used by the construction contractor for laying out the complex structures to a specified construction tolerance of ±25 mm.
A key component of the project design was to blend weir modifications with the natural surroundings of the Bow River. Doing this required utilisation of natural construction materials such as cobbles and gravels, along with large boulders with equivalent spherical diameters of 1m (1.4 tonnes) to 1.8m (8 tonnes). However as conventional hydraulic structure designs, with the desired design life of over 50 years, would typically consist of man-made materials of reinforced concrete, structural steel and/or steel sheet piles, these natural solutions still required more thought. As, for example, the design of the LWC and HWC drop structures are subjected to:
• Continuous and varying flow in a relatively turbulent hydraulic environment.
• Varying differential heads.
• Ice action/effects.
• Frequent freeze/thaw and wet/dry cycles.
To blend into the natural surroundings and to greatly increase the pace of construction, the design of the drop structures incorporated large, fieldstone boulders with macro-synthetic fibre reinforced concrete placed in the voids between the boulders for structural integrity. To allow barefoot pedestrian traffic on the structures, conventional steel fibre reinforcement was not applicable. Using boulders with concrete in the voids provided very robust, cost effective structures with minimal foundation preparation resulting in faster construction with less care of water requirements and, ultimately, less construction impact on the aquatic environment than conventional reinforced concrete hydraulic structures.
The concreted boulder structures also more closely replicate natural riffles, outcrops, and bed materials that exist within the Bow River. Plastic fibre reinforcement was utilised to increase the tensile strength and reduce thermal cracking of the concrete, provide structural integrity and significantly accelerate construction.
The project was tendered in the summer of 2008 and was awarded to DeGraaf Excavating (DGE) of Lethbridge, Alberta, for C$16.6M. Construction started in December 2008 and the whole project was fully functional on 30 April 2011, all completed on schedule and under budget.
Performance testing has been ongoing since May 2011. Swimmers, canoeists, kayakers and inflatable rafts have all travelled through and played on the drop structures. Transformation of the Calgary Bow River weir, has not only improved safety and fish passage, but created an amenity for a multitude of new recreational users.
The grand opening of the weir is set for 1 August 2012 and will be co-ordinated by the PFC. The project has won two awards of excellence from the Consulting Engineers of Alberta for Water Resources and Energy Production and for Community Development (Alberta Innovators Magazine, Spring 2012). Klohn Crippen Berger has also recently submitted the Bow River to the Canadian Consulting Engineers awards with the winner being announced at the end of July. All of this, is surely, a great achievement for a project previously labelled as a drowning machine?
For additional information, and details on other projects Klohn Crippen Berger has been involved in, please visit www.klohn.com